1. Field of the Invention
The present invention generally relates to the conversion of controlled expansion of gas under pressure to motive energy for performing work and, more particularly, is concerned with a non-contact rotary vane gas expanding apparatus for converting controlled expansion of gas under pressure to rotary motion.
2. Description of the Prior Art
The controlled expansion of gases, normally heated by combustion or nuclear energy, forms the basis for essentially all motive energy utilized by modern civilization. Steam turbines, for example, expanding steam heated by fossil or nuclear fuels in electric power plants, are the prime movers that turn the electric generators. As another example, gas turbines, which expand air heated by the hydrocarbon combustion, propel jet aircraft.
Turbo-expanders convert the momentum (kinetic energy) of the expanding gases (whose velocity arises due to differences in gas pressure occurring across the machine) to motive rotational energy. In cases where thrust is the primary goal of the machine, the remaining unconverted motive energy (kinetic) results directly in thrust. Such machines are called momentum conversion devices.
Another class of thermo-machines convert gas pressure to motive energy directly through the action of pressure differences across sealed moving mechanical surfaces of the machine. Such devices do not depend upon the conversion of momentum or kinetic energy and are known generally as positive displacement machines. A prominent example of such a device is the conventional piston engine. In this type of gas expander, high pressure combustion-heated gases trapped within the piston-cylinder force the piston to move. This linear piston motion is then converted to rotational motion through the kinematic action of the connecting rod and crankshaft.
Normally, positive displacement mechanisms are used for internal combustion engines (e.g. Otto and Diesel cycles), whereas external combustion engines (e.g. Brayton and Rankine cycles) use momentum-conversion devices. A notable but passe' exception are steam locomotive (piston) engines which use the open external combustion Rankine steam cycle.
Momentum conversion expanders (turbines) are highly non-linear devices. That is, their power output is highly dependent upon rotor speed; in fact, the cube of the rotor speed. This extreme power output nonlinearity arises because the kinetic energy contained within the flowing gas is a function of the square of the velocity of the expanding gas. Compounding this non-linearity is the fact that the mass throughput of the expanding gas varies as the first power of the rotor speed. Since the power output of the turbine is the product of the mass flow rate through the machine (the first power) and the kinetic energy content of the flowing gases (the second power), the net turbine power thus varies as the third power of rotor speed.
Such non-linearity in power output as a function of shaft speed is not a serious problem if the application of the turbine expander requires constant speed such as in an electric power generation plant. On the other hand, land vehicles demand extremes in speed changes--from idle to cruise and all intermediate conditions. Further, the torque loads on land vehicles is extremely variable due to speed, acceleration, and terrain changes. For this basic reason, gas turbines have not been and may never be compatiable with automobile propulsion. As well, it is important to realize that turbo-machinery becomes very inefficient (due in large part to inherent blade tip losses) as their size dwindles to low horsepower. Although a secondary matter, this fact also provides resistance to their use as car engines.
An attraction of external combustion power cycles such as the Rankine and Brayton cycles is their essentially steady combustion processes that are very efficient and nearly emission-free. On the other hand, the fuel-burning processes occurring within internal combustion engines are very unsteady and thus less energy-efficient and contain considerable pollutants. These emissions are so problematical that expensive catalytic converter systems have had to be developed in order to continue the widespread use of internal combustion engines in vehicles.
Thus, on the one hand, extremely efficient and clean-burning power cycles are known heretofore which must use highly non-linear and relatively large prime movers. On the other hand, less efficient and polluting internal combustion power cycles are known heretofore which operate with linear positive displacement machines suitable for vehicle propulsion.
What is desirable for land propulsion, then, is a gas expander apparatus that is suitable for use as a vehicle engine, but which can operate with clean external combustion cycles, such as Rankine or Brayton cycles.